LeetCode 128. Longest Consecutive Sequence

描述
Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

For example,
Given [100, 4, 200, 1, 3, 2],
The longest consecutive elements sequence is [1, 2, 3, 4]. Return its length: 4.

Your algorithm should run in O(n) complexity.

分析
如果允许 O(n log n) 的复杂度，那么可以先排序，可是本题要求 O(n)。 由于序列里的元素是无序的，又要求 O(n)，首先要想到用哈希表。
用一个哈希表 unordered_map

class Solution {public:    int longestConsecutive(vector<int>& nums) {        unordered_map<int, bool> used;        for (auto i : nums) used[i] = false;        int longest = 0;        for (auto i : nums) {            if (used[i] == true) continue;            int length = 1;            used[i] = true;            for (int j = i + 1; used.find(j) != used.end(); ++j) {                used[j] = true;                ++length;            }            for (int j = i - 1; used.find(j) != used.end(); --j) {                used[j] = true;                ++length;            }            longest = (length < longest) ? longest : length;        }        return longest;    }};